1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an In-Plane switching mode liquid crystal display (LCD) device and a method of manufacturing the same.
2. Discussion of the Related Art
As an information society develops, so does the demand for various types of displays. Recently, efforts have been made to research and develop various types of flat display panels, such as Liquid Crystal Display (LCD), Plasma Display Panel (PDP), Electroluminescent Display (ELD), Vacuum Fluorescent Display (VFD), and the like. An LCD is widely used as a substitution for a Cathode Ray Tube (CRT) because the LCD has the characteristics or advantages of high quality image, light weight, shallow depth, compact size, and low power consumption. An LCD is applicable for use in devices that receive display signals, such as a television, computer monitor, and the like. Various technical developments for different types of LCD have been made such that LCDs play a role as an image display in various fields. However, in order for an LCD to be used as a general display device in a variety of various fields, the LCD needs to realize a high quality image that has high resolution, high brightness, and wide screen, as well as, maintain the characteristics of light weight, shallow depth, compact size, and low power consumption.
In general, an LCD device includes an LCD panel for displaying a picture image, and a driving part for applying a driving signal to the LCD panel. The LCD panel includes first and second glass substrates that are bonded to each other with a predetermined gap therebetween, and a liquid crystal layer positioned in the gap between the first and second glass substrates. The first glass substrate (TFT array substrate) includes a plurality of gate lines, a plurality of data lines, a plurality of pixel electrodes, and a plurality of thin film transistors. The plurality of gate lines are formed on the first glass substrate at fixed intervals. The plurality of data lines are formed at fixed intervals and are perpendicular to the plurality of gate lines. The plurality of pixel electrodes, arranged in a matrix-type configuration, are respectively formed in pixel regions defined between the plurality of gate and the plurality of data lines. The plurality of thin film transistors are switched in accordance with signals on the gate lines such that signals on the respective data lines are transmitted to the respective pixel electrodes.
The second glass substrate (color filter substrate) includes a black matrix layer that excludes light from specific regions except for the pixel regions of the first substrate. An R/G/B color filter layer for displaying various colors is positioned in the black matrix layer. A common electrode is also positioned on the second glass substrate (color filter substrate) to obtain the picture image.
The LCD device is driven according to optical anisotropy and polarizability of the liquid crystal. Liquid crystal molecules have directional orientation characteristics because each of the liquid crystal molecules has a long thin shape. An applied electric field can control the alignment direction of the liquid crystal molecules. The alignment direction of the liquid crystal molecules is controlled by the electric field such that the light is refracted along the alignment direction of the liquid crystal molecules by the optical anisotropy of the liquid crystal, thereby displaying a picture image.
There are various types of liquid crystal displays. In particular, an Active Matrix Liquid Crystal Display (AM-LCD) contains thin film transistors that are respectively connected to pixel electrodes. The pixel electrodes, which are on one substrate, are arranged in a matrix and confront a common electrode, which is on the other substrate. The pixel electrodes and common electrodes drive liquid crystal molecules by applying an electric field between the substrates in a direction vertical to the substrates. The AM-LCD provides excellent resolution for displaying moving images.
FIG. 1 is an exploded perspective view illustrating a general twisted nematic (TN) mode LCD device of the related art. As shown in FIG. 1, the TN mode LCD device includes a lower substrate 1 and an upper substrate 2 bonded to each other with a predetermined gap therebetween. A liquid crystal layer 3 is positioned in the gap between the lower substrate 1 and the upper substrate 2. More specifically, the lower substrate 1 includes a plurality of gate lines 4, a plurality of data lines 5, a plurality of pixel electrodes 6, and a plurality of thin film transistors T. The plurality of gate lines 4 are formed on the lower substrate 1 in one direction at fixed intervals. The plurality of data lines 5 are formed in a direction perpendicular to the plurality of gate lines 4 at fixed intervals. A plurality of pixel regions P are defined between the plurality of gate lines 4 and the plurality of data lines 5. A plurality of pixel electrodes 6 are respectively formed in the pixel regions P. A plurality of thin film transistors T are respectively formed at crossings of the plurality of gate lines 4 and the plurality of data lines 5. The upper substrate 2 includes a black matrix layer 7 that excludes light from specific regions except for the pixel regions P, R/G/B color filter layer 8 for displaying various colors, and a common electrode 9 for displaying a picture image.
The thin film transistor T includes a gate electrode, a gate insulating layer (not shown), an active layer, a source electrode, and a drain electrode. The gate electrode for the thin film transistor T projects from the gate line 4. The gate insulating layer (not shown) is formed on an entire surface of the lower substrate. Then, the active layer of the thin film transistor T is formed on the gate insulating layer above the gate electrode. The source electrode for the thin film transistor T projects from the data line 5, and the drain electrode is formed opposite to the source electrode. The aforementioned pixel electrode 6 is connected to the drain electrode and is formed of transparent conductive metal having great transmittance, such as indium-tin-oxide (ITO).
In the aforementioned LCD device, liquid crystal molecules of the liquid crystal layer 3 positioned on the pixel electrode 6 are aligned in response to a signal applied via the thin film transistor T. Light transmittance is controlled according to alignment of liquid crystal, thereby displaying a picture image. In other words, an LCD panel drives the liquid crystal molecules by an electric field perpendicular to the lower and upper substrates. This method obtains great transmittance and high aperture ratio. Also, it is possible to prevent liquid crystal cells from being damaged by static electricity since the common electrode 9 of the upper substrate 2 serves as the ground. However, by driving the liquid crystal molecules with an electric field that is perpendicular to the lower and upper substrates, it is difficult to obtain a wide viewing angle.
In order to overcome this narrow viewing angle problem, an In-Plane switching mode LCD device is used. A related art In-Plane Switching (IPS) mode LCD device will be described with reference to the FIGS. 2–6. FIG. 2 is a cross-sectional view illustrating electric field and alignment direction of liquid crystal in a related art In-Plane switching mode LCD device.
In the related art In-Plane switching mode LCD device, as shown in FIG. 2, a common electrode 13 and a pixel electrode 15 are formed on the same plane of a lower substrate 10. Then, the lower substrate 10 is bonded to an upper substrate 20 with a predetermined gap therebetween. The liquid crystal 3 is positioned in the gap between the lower substrate 10 and the upper substrate 20. The liquid crystal 3 is driven by an electric field formed between the common electrode 13 and the pixel electrode 15 on the lower substrate 10.
FIG. 3A and FIG. 3B illustrate the alignment direction of liquid crystal when a voltage is turned off/on in the related art In-Plane switching mode LCD device.
FIG. 3A illustrates the related art In-Plane switching mode LCD device when the voltage is turned off. That is, an electric field, which is parallel to the lower and upper substrates, is not applied to the common electrode 13 or the pixel electrode 15. Accordingly, there is no change in alignment of the liquid crystal 3.
FIG. 3B illustrates the related art In-Plane switching mode LCD device when the voltage is turned on. That is, an electric field, which is parallel to the lower and upper substrates, is generated between the common electrode 13 and the pixel electrode 15. Accordingly, alignment of the liquid crystal 3 is changed. In more detail, the alignment of the liquid crystal 3 is twisted at 45° as compared to the alignment of liquid crystal when the voltage is turned off. During the on state, the twisted direction of liquid crystal adjacent to the lower substrate is identical to the horizontal direction of the common electrode 13 and the pixel electrode 15.
As mentioned above, the related art In-Plane switching mode LCD device has the common electrode 13 and the pixel electrode 15 on the same plane. Thus, it has advantageous characteristics, such as a wide viewing angle. For example, along a front direction of the In-Plane switching mode LCD device, a viewer can have a viewing angle of 70° in all directions (i.e., lower, upper, left, and right directions). Furthermore, the related art In-Plane switching mode LCD device has simplified fabrication process steps, and reduced color shift. However, the related art In-Plane switching mode LCD device has the problems of low light transmittance and low aperture ratio since the common electrode 13 and the pixel electrode 15 are formed on the same substrate.
In the meantime, the related art In-Plane switching mode LCD device forms the different electric fields according to arrangement of the respective electrodes. FIG. 4A and FIG. 4B illustrate the electric field according to arrangements of the respective electrodes.
Referring to FIG. 4A, the common electrode 13 and the pixel electrode 15 are positioned at a predetermined interval so as to be completely in between the lower and upper substrates (not shown). When a voltage signal is applied to the common electrode 13 and the pixel electrode 15 so as to generate a voltage difference between the two electrodes, an electric field, parallel to the substrates, having no distortion is generated directly between the common electrode 13 and the pixel electrode 15.
Referring to FIG. 4B, the common electrode 13 and the pixel electrode 15 are formed on only one of the lower and upper substrates (not shown) at a predetermined interval therebetween so as to leave a space between the electrodes and the other substrate. When a voltage signal is applied so as to generate a voltage difference between the two electrodes 13 and 15, an electric field parallel to the substrate above the common electrode 13 and the pixel electrode 15 have curved distortions at portions adjacent to the common electrode 13 and the pixel electrode 15.
The arrangement of the respective electrodes shown in FIG. 4A is ideal for the drive of liquid crystal in the In-Plane switching mode LCD device. However, the arrangement of the electrodes shown in FIG. 4A has difficulties in the manufacturing process, such as positioning the liquid crystal in the gap between the substrates. That is, when forming the two electrodes according to the arrangement shown in FIG. 4A, it is impossible to inject the liquid crystal between the lower and upper substrates. Meanwhile, as shown in FIG. 4B, in case of the general In-Plane switching mode LCD device forming the common electrode 13 and the pixel electrode 15 on any one of the lower and upper substrates, a great voltage difference between the common electrode 13 and the pixel electrode 15 is required to form the electric field parallel to the lower and upper substrates adjacent to an upper substrate from which the electrodes are separated, thereby causing the problem of increased power consumption.
Hereinafter, alignment of liquid crystal before and after applying the voltage to the electrodes of the related art In-Plane switching mode LCD device will be described as follows. FIG. 5 is a plane view illustrating the related art In-Plane switching mode LCD device. FIG. 6A and FIG. 6B are cross-sectional views illustrating alignment of liquid crystal before and after applying the voltage along the line I—I′ of FIG. 5. As shown in FIG. 5, FIG. 6A and FIG. 6B, the related art In-Plane switching mode LCD device includes a lower substrate 10, an upper substrate 20 facing the lower substrate 10, and a liquid crystal layer formed between the lower substrate 10 and the upper substrate 20.
Referring to FIG. 5, a gate line 11 and a data line 12 cross each other on the lower substrate 10 to define a pixel region. Then, a common electrode 13 and a pixel electrode 15 are formed at a predetermined interval within the pixel region. A thin film transistor TFT is formed within the pixel region on the lower substrate 10. The thin film transistor TFT includes a gate electrode 11a, a gate insulating layer (for reference, ‘14’ of FIG. 6A and FIG. 6B), a semiconductor layer 18, a source electrode 12a and a drain electrode 12b. The gate electrode 11a projects from the gate line 11. The gate insulating layer is formed on an entire surface of the lower substrate 10 including the gate electrode 11a. The semiconductor layer 18 is formed overlapping the gate electrode 11a. The source electrode 12a projecting from the data line 12 is formed at a predetermined interval from the drain electrode 12. The source electrode 12a and drain electrode 12b are formed at both sides of the semiconductor layer 18. The drain electrode 12b of the thin film transistor TFT is connected with the pixel electrode 15.
The common electrode 13 is formed at a predetermined interval from the pixel electrode 15. The common electrode 13 may be simultaneously formed when forming the gate line 11 or the data line 12. In the drawings, the common electrode 13 is formed in the same layer as the data line 12. Furthermore, a passivation layer 16 is formed between the data line 12 and the pixel electrode 15. The passivation layer (for reference, ‘16’ of FIG. 6A and FIG. 6B) is formed of the same material as the gate insulating layer 14, such as an inorganic insulating layer of SiNx or SiOx, or an organic insulating layer of acryl, polyimide, BenzoCycloButene (BCB) or photo polymer.
Subsequently, a first alignment layer 17 is formed on the entire surface of the lower substrate 10 including the passivation layer 16 and the pixel electrode 15. Thus, when the common electrode 13 receives a voltage signal from a common line 19, and a voltage signal is applied to the pixel electrode 15 through the drain electrode 12b, an electric field parallel to the substrates is generated to drive the liquid crystal.
The upper substrate 20 includes a black matrix layer 21, a color filter layer 22, and a second alignment layer 23. The black matrix layer 21 is formed to correspond to specific regions of the lower substrate except for the pixel regions, thereby preventing light leakage in the specific regions. A color filter layer 22 is formed within the black matrix layer 21 to obtain R/G/B color throughout the pixel regions. The second alignment layer 23 is formed to so as to define initial orientation of the liquid crystal.
The first alignment layer 17 and the second alignment layer 23 are respectively formed on the entire surfaces of the lower substrate 10 and the upper substrate 20 to define the initial orientation of the liquid crystal. Accordingly, liquid crystal molecules adjacent to the first alignment layer 17 and the second alignment layer 23 are oriented in accordance with the alignment direction of the first and second alignment layers.
Referring to FIG. 6A, before applying the voltage, the orientation of the liquid crystal is determined by the rubbing direction of the first alignment layer 17 and the second alignment layer 23, respectively formed on the lower substrate 10 and upper substrate 20. Accordingly, as shown in FIG. 6A, the liquid crystal molecules positioned along the vertical direction of the common electrodes 13 and pixel electrodes 15 have similar round shapes. As shown in FIG. 6A, before applying the voltage, the related art In-Plane switching mode LCD device operates in a Normally Black, whereby it is impossible to perform light transmission.
Referring to FIG. 6B, when the voltage is applied to the common electrode 13 and the pixel electrode 15, the electric field generates between the common electrode 13 and the pixel electrode 15 formed on the same substrate. Thus, the liquid crystal molecules are aligned along the electric field formed between the common electrode 13 and the pixel electrode 15. In this case, the liquid crystal molecules positioned along the vertical direction to the common electrode 13 and the pixel electrode 15d have a long elliptical shape in this view rather than the circular shape of the original form of liquid crystal molecule since the direction of the liquid crystal molecules has been changed.
After applying the voltage to the general In-Plane switching mode LCD device, the light is transmitted so that a white state is displayed. Since there are liquid crystal molecules positioned where the electric field divides adjacent to the common electrode 13 and the pixel electrode 15, it is difficult to move the liquid crystal molecules to the predetermined direction by applying the voltages to the electrodes. Thus, in the display mode, disinclination generates at the portions of the electrodes where the electric field divides. To address this disinclination, the common electrode 13 and the pixel electrode 15 are formed of metal or ITO/metal alloy to prevent light leakage through the common electrode 13 and the pixel electrode 15.
However, the related art In-Plane switching mode LCD device has the following disadvantages. In the related art In-Plane switching mode LCD device, the liquid crystal is driven by the electric field parallel to the lower and upper substrates between the common electrode and the pixel electrode. At this time, the common electrode and the pixel electrode are formed on any one of the lower and upper substrates. Accordingly, the electric field parallel to the lower and upper substrates is only generated adjacent to the one substrate having the common and pixel electrodes thereon. That is, in order to drive the liquid crystal molecules adjacent to the other substrate having no common and pixel electrodes thereon, it is necessary to greatly increase voltage difference to the common and pixel electrodes, thereby causing the problem of increase in power consumption.